Role of agonistic autoantibodies against type-1 angiotensin II receptor in the pathogenesis of retinopathy in preeclampsia

Reliability of Rodent and Rabbit Models in Preeclampsia Research

In vivo studies on the pathology of gestation, including preeclampsia, often use small mammals such as rabbits or rodents, i.e., mice, rats, hamsters, and guinea pigs. The key advantage of these animals is their short reproductive cycle; in addition, similar to humans, they also develop a haemochorial placenta and present a similar transformation of maternal spiral arteries. Interestingly, pregnant dams also demonstrate a similar reaction to inflammatory factors and placentally derived antiangiogenic factors, i.e., soluble fms-like tyrosine kinase 1 (sFlt-1) or soluble endoglin-1 (sEng), as preeclamptic women: all animals present an increase in blood pressure and usually proteinuria. These constitute the classical duet that allows for the recognition of preeclampsia. However, the time of initiation of maternal vessel remodelling and the depth of trophoblast invasion differs between rabbits, rodents, and humans. Unfortunately, at present, no known animal replicates a human pregnancy exactly, and hence, the use of rabbit and rodent models is restricted to the investigation of individual aspects of human gestation only. This article compares the process of placentation in rodents, rabbits, and humans, which should be considered when planning experiments on preeclampsia; these aspects might determine the success, or failure, of the study. The report also reviews the rodent and rabbit models used to investigate certain aspects of the pathomechanism of human preeclampsia, especially those related to incorrect trophoblast invasion, placental hypoxia, inflammation, or maternal endothelial dysfunction.

Angiotensin and Endothelin Receptor Structures With Implications for Signaling Regulation and Pharmacological Targeting

In conjunction with the endothelin (ET) type A (ETAR) and type B (ETBR) receptors, angiotensin (AT) type 1 (AT1R) and type 2 (AT2R) receptors, are peptide-binding class A G-protein-coupled receptors (GPCRs) acting in a physiologically overlapping context. Angiotensin receptors (ATRs) are involved in regulating cell proliferation, as well as cardiovascular, renal, neurological, and endothelial functions. They are important therapeutic targets for several diseases or pathological conditions, such as hypertrophy, vascular inflammation, atherosclerosis, angiogenesis, and cancer. Endothelin receptors (ETRs) are expressed primarily in blood vessels, but also in the central nervous system or epithelial cells. They regulate blood pressure and cardiovascular homeostasis. Pathogenic conditions associated with ETR dysfunctions include cancer and pulmonary hypertension. While both receptor groups are activated by their respective peptide agonists, pathogenic autoantibodies (auto-Abs) can also activate the AT1R and ETAR accompanied by respective clinical conditions. To date, the exact mechanisms and differences in binding and receptor-activation mediated by auto-Abs as opposed to endogenous ligands are not well understood. Further, several questions regarding signaling regulation in these receptors remain open. In the last decade, several receptor structures in the apo- and ligand-bound states were determined with protein X-ray crystallography using conventional synchrotrons or X-ray Free-Electron Lasers (XFEL). These inactive and active complexes provide detailed information on ligand binding, signal induction or inhibition, as well as signal transduction, which is fundamental for understanding properties of different activity states. They are also supportive in the development of pharmacological strategies against dysfunctions at the receptors or in the associated signaling axis. Here, we summarize current structural information for the AT1R, AT2R, and ETBR to provide an improved molecular understanding.

Molecular Effects of Auto-Antibodies on Angiotensin II Type 1 Receptor Signaling and Cell Proliferation

The angiotensin II (Ang II) type 1 receptor (AT1R) is involved in the regulation of blood pressure (through vasoconstriction) and water and ion homeostasis (mediated by interaction with the endogenous agonist). AT1R can also be activated by auto-antibodies (AT1R-Abs), which are associated with manifold diseases, such as obliterative vasculopathy, preeclampsia and systemic sclerosis. Knowledge of the molecular mechanisms related to AT1R-Abs binding and associated signaling cascade (dys-)regulation remains fragmentary. The goal of this study was, therefore, to investigate details of the effects of AT1R-Abs on G-protein signaling and subsequent cell proliferation, as well as the putative contribution of the three extracellular receptor loops (ELs) to Abs-AT1R signaling. AT1R-Abs induced nuclear factor of activated T-cells (NFAT) signaling, which reflects Gq/11 and Gi activation. The impact on cell proliferation was tested in different cell systems, as well as activation-triggered receptor internalization. Blockwise alanine substitutions were designed to potentially investigate the role of ELs in AT1R-Abs-mediated effects. First, we demonstrate that Ang II-mediated internalization of AT1R is impeded by binding of AT1R-Abs. Secondly, exclusive AT1R-Abs-induced Gq/11 activation is most significant for NFAT stimulation and mediates cell proliferation. Interestingly, our studies also reveal that ligand-independent, baseline AT1R activation of Gi signaling has, in turn, a negative effect on cell proliferation. Indeed, inhibition of Gi basal activity potentiates proliferation triggered by AT1R-Abs. Finally, although AT1R containing EL1 and EL3 blockwise alanine mutations were not expressed on the human embryonic kidney293T (HEK293T) cell surface, we at least confirmed that parts of EL2 are involved in interactions between AT1R and Abs. This current study thus provides extended insights into the molecular action of AT1R-Abs and associated mechanisms of interrelated pathogenesis.

Receptors | Angiotensin Receptors

The renin-angiotensin-aldosterone system (RAS) is a vital hormone-receptor system that regulates cardiovascular and renal functions. In this article, we discuss exciting new findings in the RAS field. Recently solved active state crystal structures of Angiotensin II type 1 (AT1R) and type 2 receptor (AT2R) helped in understanding receptor activation mechanisms in detail. Also, considerable attention is given to the developments in characterizing the counter-regulatory RAS axis due to current hope for harnessing this axis for the development of protective therapies against various cardiovascular diseases. We describe the RAS component, angiotensin-converting enzyme 2 (ACE2) functioning as cellular entry receptor for the causative agent of COVID-19 pandemic, SARS-CoV-2. Altogether, these discoveries paved the way for developing novel therapies targeting different components of the RAS in the future.

Angiotensin receptor autoantibodies as exposures that modify disease progression: Cross sectional, longitudinal and in vitro studies of prostate cancer

Circulating angiotensin type I receptor (AT1R) agonistic autoantibodies (AT1RaAbs) that bind and chronically activate the receptor have been associated with a number of diseases suggesting that while the autoantibodies are not necessarily causative they may promote disease progression. The prostate has a local renin angiotensin system. The current study examines associations between AT1RaAbs and prostate cancer (PCA), disease-free survival (DFS), overall survival (OS) and AT1RaAb effects on PCA cell phenotype. In a cross-sectional set of serum obtained from 151 men diagnosed with PCA, nonmalignant prostate disease or no disease, higher serum AT1RaAb levels were associated with PCA and non-organ confined PCA. The odds ratio for PCA was 6.3 (95% confidence interval 2.2 to 18) for a positive 1:1600 titer and 18 (95% confidence interval 6.9 to 45) at AT1RaAb levels > 1.04 μg/ml, (p < 0.0001). In a longitudinal set of pre-diagnosis samples from 109 men, DFS hazard ratios of 2.2 (95% confidence interval 1.4 to 3.5) and 1.6 (95% confidence interval 1.0 to 2.5) for most proximal to diagnosis and most distal to diagnosis samples, respectively, were found for high versus low AT1RaAb groups. Hazard ratios for OS in most proximal and distal samples were 2.4 (95% confidence interval 1.6 to 3.6) and 1.8 (95% confidence interval 1.1 to 2.8), respectively. Accelerated failure modeling of survival indicated that a 1 μg/ml increase in AT1RaAb levels was associated with a reduction of DFS and OS by 20% at the most proximal time point and by 15% at the most distal time points. Adjusting for age, did not affect the association with DFS in proximal samples but changed distal time point DFS and OS to a 10% decrease for every 1 μg/ml increase in AT1RaAb. Additional adjustments for body mass index, systolic blood pressure and prostate-specific antigen did not appreciably alter these associations. AT1RaAb treatment of PC3, DU145, and LNCaP cells significantly increased the maximal growth rate approximately 2-fold and invasiveness approximately 3-fold. Conclusions: These observations provide evidence supporting AT1RaAbs as exposures that may modify prostate cancer progression and indicate they may be predictive markers for risk stratification.

Angiotensin II type i receptor agonistic autoantibodies induces apoptosis of cardiomyocytes by downregulating miR21 in preeclampsia: a mechanism study

Angiotensin II type I receptor agonistic autoantibodies (AT₁-AA) in the plasma of preeclampsia patients can induce apoptosis of cardiomyocytes, and microRNA-21 (miR-21) can exert a protective effect on cardiomyocytes. But whether the pro-apoptotic effect of AT₁-AA is associated with miR-21 is unclear. The objective of the present study was to explore whether AT₁-AA induced cardiomyocyte apoptosis was related to its inhibitory of miR-21 expression. In vivo studies, the pregnant rats were divided into two groups: Sham group, Model group. The pathology, cell apoptosis, and relative protein expressions were evaluated by hematoxylin and eosin staining, and Western blot assay. The expression of microRNA was detected by gene microarray. In the cell experiment, the neonatal rat cardiomyocytes were divided into four groups: NC group, AT₁-AA group, and miR-21 group and AT₁-AA+miR-21 group. The cell apoptosis and relative proteins' expressions were measured by flow cytometry and Western blot assay. Results: Compared with the Sham group, miR-21 in the cardiac tissue of the model group was downregulated significantly; the expression of p-JNK, Bax and caspases-3 was increased, the expression of Bcl-2 was decreased, and the Bcl-2/Bax ratio became smaller. The expression of miR-21 in AT₁-AA treated cardiomyocytes was only 52% of the control group, with an apoptosis rate of 32.6%. In addition, the expression of pPTEN, pAKT and pFOXO3a in the model group was significantly higher than that in the NC group. The cardiomyocyte apoptosis rate in miR-21 overexpression group was only 23.7%, which was higher than that in the NC group, but significantly lower than that in AT₁-AA group. PTEN, AKT and FOXO3a phosporylation in miR-21 overexpression group was also lower than that in AT₁-AA group. AT₁-AA induced cardiomyocyte apoptosis by downregulating miR-21, and the PTEN/AKT/FOXO3a signal transduction pathway participated in this process. The result of the present study suggests that miR-21 may prove to be a new target for the diagnosis and treatment of preeclampsia and other cardiovascular diseases.

Functional relevance of promoter CpG island of human Angiotensin II type 1 receptor (AT1R) gene

Angiotensin II type 1 receptor can activate number of signalling pathways upon stimulation and consequently its involvement in cancer progression have also been revealed. But which epigenetic mechanisms are involved in its regulation, need to be further explored. In-silico analysis revealed a promoter CpG island (CGI) which was cloned and assayed for functional activity using reporter gene system. The effect of methylation on this CGI was analysed through varying degree of methyltransferase treatment of cloned fragment. Results thus obtained were validated by direct sequencing. To further establish the status of this effect, in-vivo analysis was done through screening of methylation pattern in the targeted region among hypertensives (HTN) and normotensives (NTN) using PCR-Bisulphite sequencing. Additionally, clinical details of all participants, biochemical parameters and lifestyle related information was also collected and statistically evaluated. Reporter gene assay assigned functional activity to the cloned promoter CpG island. Increased dose and durations of methyltransferase treatment decreased the expression of reporter gene indicating the role of promoter DNA methylation. Among all the human samples screened, only one of the HTN individual was found to have single hemi-methylated CpG site at a position which happens to be a part of Sp1 transcription factor binding site. To conclude, CpG island in the promoter region of AT1R (CpG.P.AT1R) gets regulated through epigenetic mechanism of DNA methylation.

Effects and molecular mechanisms of AT1-AA in retinopathy of preeclampsia

Preeclampsia not only seriously endangers maternal and fetal health during pregnancy but may incur many sequelae in postpartum women such as reduced visual acuity. Agonistic autoantibodies to the angiotensin II type I receptor (AT1-AA) is closely associated with preeclampsia. The aim of the present study is to determine whether AT1-AA is associated with retinal impairment during the course of preeclampsia. A preeclampsia model was established by injecting AT1-AA into pregnant rats via the tail vein. Changes in the retinal histological structure were observed. Cell apoptosis and cytokines including reactive oxygen species (ROS), as well as apoptosis-related proteins such as Bcl-2, Bax, and caspase-3 were detected. In addition, flash electroretinograms obtained at different postpartum days were analyzed. Compared with the control group, the retinal structure became edematous and the cell density was reduced significantly in preeclampsia group. The cell apoptosis rate was increased significantly. In addition, the content of ROS, the levels of Bax and caspase-3 in the retina were increased, while the content of Bcl-2 was reduced significantly. Continuous observation of the electroretinograms showed loss of retinal ganglion cells postpartum. The present study demonstrated that AT1-AA induced retinal cell apoptosis by promoting ROS release and activating caspase, suggesting that the increased postpartum susceptibility of preeclamptic women to retinopathy is related to AT1-AA-induced cell apoptosis.

Roles and possible mechanisms of autoantibodies against the angiotensin AT1 receptor in vascular calcification of rats

The aim of this study was to investigate the effect of AT1-AAs on vascular calcification. Wistar rats were immunized with synthetic peptides corresponding to the second extracellular loop of AT1 receptor. The titer of AT1-AAs in rat serum, SBP, and HR were detected weekly. Histological analysis and biochemical parameters were measured 4 months after initial immunization. The level of osteopontin and osteocalcin was measured by Western blot analysis. The results showed that the titer of AT1-AAs, SBP, and HR were all increased significant 4 weeks after initial immunization. Compared with the control group, the contractile force of the aortic ring of the immunized group to phenylephrine was significantly higher, and relaxation function was significantly reduced. The ALP activity and protein levels of osteoporotin and osteocalcin were increased in the aortic tissue of the immunized group. Histological examination showed varying degrees of calcification within each cell layer with von Kossa staining in the immunized group. Losartan treatment not only significantly lowered SBP and HR to the similar level of the control group but inhibited the ALP activity and protein levels of osteoporotin and osteocalcin. Our study demonstrated that AT1-AAs contributed to the progression of vascular calcification, suggesting that AT1-AAs may play a vital role in the occurrence of cardiovascular disease in patients with severe hypertension.

Ketone body 3-hydroxybutyrate mimics calorie restriction via the Nrf2 activator, fumarate, in the retina

Calorie restriction (CR) being the most robust dietary intervention provides various health benefits. D-3-hydroxybutyrate (3HB), a major physiological ketone, has been proposed as an important endogenous molecule for CR. To investigate the role of 3HB in CR, we investigated potential shared mechanisms underlying increased retinal 3HB induced by CR and exogenously applied 3HB without CR to protect against ischemic retinal degeneration. The repeated elevation of retinal 3HB, with or without CR, suppressed retinal degeneration. Metabolomic analysis showed that the antioxidant pentose phosphate pathway and its limiting enzyme, glucose-6-phosphate dehydrogenase (G6PD), were concomitantly preserved. Importantly, the upregulation of nuclear factor erythroid 2 p45-related factor 2 (Nrf2), a regulator of G6PD, and elevation of the tricarboxylic acid cycle's Nrf2 activator, fumarate, were also shared. Together, our findings suggest that CR provides retinal antioxidative defense by 3HB through the antioxidant Nrf2 pathway via modification of a tricarboxylic acid cycle intermediate during 3HB metabolism.

Role and mechanism of AT1-AA in the pathogenesis of HELLP syndrome

HELLP syndrome remains a leading cause of maternal and neonatal mortality and morbidity worldwide, which symptoms include hemolysis, elevated liver enzymes and low platelet count. The objective of this study was to determine whether HELLP is associated with AT1-AA. The positive rate and titer of AT1-AA in plasma from pregnant women were determined, and the correlation of AT1-AA titer with the grade of HELLP was analyzed. A HELLP rat model established by intravenous injection of AT1-AA. Our experimental results show the AT1-AA titer and positive rate were significantly higher in HELLP group, and AT1-AA titer were positively correlated with the level of TNF-α and ET-1 in plasma and the grade of HELLP syndrome. The results of animal experiments showed that the typical features of HELLP in the pregnant rats after AT1-AA injection. The levels of TNF-α and ET-1 in plasma and liver tissue were significantly increased in AT1-AA-treated rats compared with control rats. The HELLP syndrome induced by AT1-AA was attenuated markedly after administration of losartan. These data support the hypothesis that one the potential pathway that AT1-AA induce damage to capillary endothelial cells and liver during pregnancy is through activation of TNF-α and ET-1.

Current topics in angiotensin II type 1 receptor research: Focus on inverse agonism, receptor dimerization and biased agonism

Although the octapeptide hormone angiotensin II (Ang II) regulates cardiovascular and renal homeostasis through the Ang II type 1 receptor (AT1R), overstimulation of AT1R causes various human diseases, such as hypertension and cardiac hypertrophy. Therefore, AT1R blockers (ARBs) have been widely used as therapeutic drugs for these diseases. Recent basic research and clinical studies have resulted in the discovery of interesting phenomena associated with AT1R function. For example, ligand-independent activation of AT1R by mechanical stress and agonistic autoantibodies, as well as via receptor mutations, has been shown to decrease the inverse agonistic efficacy of ARBs, though the molecular mechanisms of such phenomena had remained elusive until recently. Furthermore, although AT1R is believed to exist as a monomer, recent studies have demonstrated that AT1R can homodimerize and heterodimerize with other G-protein coupled receptors (GPCR), altering the receptor signaling properties. Therefore, formation of both AT1R homodimers and AT1R-GPCR heterodimer may be involved in the pathogenesis of human disease states, such as atherosclerosis and preeclampsia. Finally, biased AT1R ligands that can preferentially activate the β-arrestin-mediated signaling pathway have been discovered. Such β-arrestin-biased AT1R ligands may be better therapeutic drugs for cardiovascular diseases. New findings on AT1R described herein could provide a conceptual framework for application of ARBs in the treatment of diseases, as well as for novel drug development. Since AT1R is an extensively studied member of the GPCR superfamily encoded in the human genome, this review is relevant for understanding the functions of other members of this superfamily.